In flight, a gun fired (non-self-propelled) projectile, such as a bullet, shell, etc. forms a partial vacuum adjacent to the rear or base of the projectile. This partial vacuum or low pressure area creates a force which acts on the projectile in a direction opposite its motion, thereby lessening the flight velocity of the projectile. This force is commonly referred to as "base-drag".
In an attempt to reduce base-drag, various pyrotechnic materials have been loaded into base cavities in the projectiles and ignited at the time of discharge. The products from the burning of the pyrotechnic material at least partially offset the vacuum generated by the projectile, reducing base-drag and aiding the flight of the projectile. Optimally, an ambient pressure equal to atmospheric is created by the products emitted by the burning of the pyrotechnic materials in the projectile. Pressures exceeding atmospheric could undesirably cause the bullet or projectile to be deflected from its intended path. In order to efficiently utilize the base flow effect, it should occur during a considerable part of the flight time. The object of base-bleed charge is not to create any effect comparable with the effect of a rocket motor, i.e., to cause an additional reaction force or thrust acting on the projectile in the direction of the trajectory, but merely to arrange for a low pressure flow of mainly gaseous combustion products, thus reducing the base-drag acting on the projectile during flight.
In U.S. Pat. No. 4,213,393, Gunners et al disclose reducing base-drag of a gun projectile by ejecting a flow of gas into and liberating heat in the near wake zone of the projectile base. The base-drag reduction means involves a combustion chamber, nozzle and propellant grain means of a fuel rich composition located in the combustion chamber and presenting a burning area operable to be ignited upon exit from the muzzle of the gun. According to Gunners et al, in flight the pressure in the combustion chamber only slightly exceeds the pressure at the base (0.01-0.5 bar). Further, this patent discloses that the total area of the base, the area of the restricted gas outlet and the dimensionless mass flow rate of the ejected gas are to be in a defined mathematical relationship.
Various pyrotechnic materials have been suggested for use to reduce base-drag, including liquid and solid materials. The materials for use in the base of projectiles are commonly referred to as "fumers" or "base-bleed" materials. It is desired that such materials be more easily ignited, have a slower burning rate and burn at a lower temperature than conventional fuel mixes used, e.g., in rocket motors. Solid fumers generally use dry pyrotechnic powder components that must be formed into a consolidated state, i.e., a propellant set, by mixing with bonding agents. These solid, molded fumers are generally cylindrical in shape and have an axial combustion channel. Often they are insulated at their external surfaces in order that an orderly burn-up from the inside of the channel to the outside, with the gases venting out the channel, is guaranteed.
Puchalski, in U.S. Pat. No. 3,886,009, discloses a solid pyrotechnic mixture used to reduce base-drag which requires stoichiometric portions of fuel and oxidizer. It contains magnesium powder as the fuel and strontium nitrate as the oxidizer. In addition, it contains calcium resinate as a binder and gelatin to provide low molecular weight products upon ignition. According to the patent's teachings, the mixture should burn at high yield temperatures of at least 2700.degree. C. in order to ensure the production of sufficient pressure within the base area during flight. It is further taught in that patent that the combustion products are primarily gaseous although some liquid and solid particles are also emitted into the void behind the projectile during the burning process.
Boggs et al, in U.S. Pat. No. 4,130,061, teaches that a fumer exhibiting reduced base-drag can be achieved without the high yield temperatures mentioned above by utilizing a pyrotechnic material comprising fuel (metal powder, resin and plasticizer) and oxidizers (alkaline earth metal compound and oxidizer) in greater than stoichiometric amounts. It is taught in this patent that the excess fuel composition has the dual function of not only providing an appropriate initial reaction but also providing a hot ready-state where excess fuel-rich combustion products will secondarily react with oxygen in the atmosphere to rapidly fill the void immediately adjacent the base of the projectile, i.e., the near wake zone.
Ward, in U.S. Pat. No. 4,302,259, teaches an improved pyrotechnic fuel composition comprising a metal fuel and a conventional pyrotechnic oxidizer, wherein the improvement comprises the use of a metal hydride as the fuel ingredient. According to this patent, employing, e.g., MgH.sub.2 instead of Mg produces a pyrotechnic composition which readily ignites, has a lower burning rate for increased luminous intensity, and desirably has a higher specific impulse.
Klohn et al, in U.S. Pat. No. 4,452,145, teach a process for the manufacture of insulated propellant sets for base-bleed gas generators in order to provide that the sets are consumed during burning from inside to outside as desired. According to the improvement of the invention, the propellant is introduced into an insulating envelope which contains a thermoplastically deformable elastomer. The propellant sets consist of a propellant containing one or more solid substances, at least one of which is an oxidant, and a binder being a thermoplastically deformable elastomer. During compression of the envelope about the propellant at increased temperatures, an intimate bond is formed between the binder of the propellant and the envelope containing the same type of thermoplastically deformable elastomer as main component. An exemplary propellant disclosed to be a mixture comprising ammonium perchlorate as the oxidant, nitroguanidine as the fuel, trioctyl phosphate as a plasticizer and a thermoplastically deformable elastomer like butadiene-styrene copolymer as the binder.
However, solid base-bleed, pyrotechnic compositions may suffer from disadvantages such as high cost, difficulty in processing and unsuitability for high gas rate production over an extended period of time. Often such solid compositions are high burning temperature materials which require special fabrication of the chamber in which they are placed in the projectile. Still further, the compositions may produce such liquid and solid combustion products which, in addition to making the composition not clean burning, may decrease the efficiency of the composition. Other fumers of the castable type also may be undesirable because of their inconsistent burning rate and performance.
Kurtz, in U.S. Pat. No. 4,497,676, discloses Preparation of a composition useful with firearms as a substitute for a wide variety of gunpowder formulations, including black powder, as well as being suitable for consumable cartridges and cartridge cases and solid propellent applications. The composition comprises between 50-75% of an inorganic nitrate, such as potassium nitrate and ammonium nitrate, and between 25-50% by weight of an organic acid: erythorbic acid and ascorbic acid and mixtures thereof. The composition disclosed by Kurtz is also suitable as the base-bleed, pyrotechnic composition of the present invention. Kurtz does not teach employing such composition in base-bleed systems of non-self-propelled projectiles to reduce the base-drag thereof.